I hope people don't mind me offering this as a token of my gratitude, should someone be able to resolve this-

I'm completely stuck now and feel powerless, unable to finish this edit because of this technical problem...

Basically, when I apply Time Remapping to a nested sequence, it becomes black from about halfway through, onwards. If I disable Mercury Playback Engine, it solves this issue and the blackness is replaced by the remainder of the clip but other problems happen (edge feathering suddenly isn't possible[!?] and obviously performance)

Not sure if it's directly related but if I apply a new Edge feather it curiously slips clips back or forward a few frames..

Not much you can do. There has been a bug in CS6 from the beginning with speed changes in nested sequences and it is obviously so difficult to solve that Adobe has deferred that to sometime in the future. Sometime is not defined.

I think there should be a law stipulating that companies like Adobe must make it clear when a feature in their (pretty expensive) product doesn't work properly, so people like me don't stuck in a situation where they have a deadline they risk not meeting because the software doesn't work as advertised...

I'll have a think about a workaround I guess.. maybe something in After Effects..